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 A196104 Repdigit semiprimes (semiprimes composed of identical digits). 4
 4, 6, 9, 22, 33, 55, 77, 111, 1111, 11111, 1111111, 11111111111, 11111111111111111, 2222222222222222222, 3333333333333333333, 5555555555555555555, 7777777777777777777, 22222222222222222222222, 33333333333333333333333, 55555555555555555555555 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS A semiprime can be repdigit (base 10) in only three ways. It can be a single-digit semiprime, a repunit semiprime (A102782), or a repunit prime times a prime digit {2, 3, 5, 7}. Occurs in proof that the sequence is infinite in which a(n) is the least semiprime > a(n-1) such that a(n) has no digit in common with a(n-1). - Jonathan Vos Post; corrected by Max Alekseyev, Sep 14 2022 LINKS Max Alekseyev, Table of n, a(n) for n = 1..35 FORMULA Union of {4, 6, 9}, A102782, 2*A004022, 3*A004022, 5*A004022, and 7*A004022. - Jonathan Vos Post and R. J. Mathar, Oct 27 2011 EXAMPLE a(12) = 11111111111 = 21649 * 513239 is semiprime. MAPLE with(numtheory):for n from 1 to 23 do:for b from 1 to 9 do:x:=(((10^n)- 1)/9)*b:if bigomega(x)=2 then printf(`%d, `, x):else fi:od:od: MATHEMATICA Select[FromDigits/@Flatten[Table[PadRight[{}, i, n], {i, 25}, {n, 9}], 1], PrimeOmega[ #] ==2&] (* Harvey P. Dale, Mar 11 2019 *) PROG (PARI) print1("4, 6, 9"); for(n=1, 20, t=10^n\9; if(bigomega(t)==2, print1(", "t)); if(isprime(t), forprime(p=2, 7, print1(", "p*t)))) \\ Charles R Greathouse IV, Oct 27 2011 CROSSREFS Subsequence of A046328. Except for initial terms, subsequence of A116063. Cf. A000042, A001358, A004023, A046413, A102782. Sequence in context: A118690 A084994 A046328 * A046376 A229129 A272436 Adjacent sequences: A196101 A196102 A196103 * A196105 A196106 A196107 KEYWORD nonn,base AUTHOR Michel Lagneau, Oct 27 2011 EXTENSIONS Edited by Max Alekseyev, Sep 14 2022 STATUS approved

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Last modified August 8 04:35 EDT 2024. Contains 375018 sequences. (Running on oeis4.)